There has been a demand to improve the power-generation efficiency of solar power generators that will lead to a reduction in manufacturing costs. One method of improving the power-generation efficiency is to form an uneven structure on the surface of a silicon substrate of the solar power generator that allows the silicon substrate to efficiently absorb more sunlight. This uneven structure referred to as “texture” is a collective term for microasperities formed on the surface of the silicon substrate. An uneven structure, for example, is known that has convex portions in the form of quadrangular pyramids each surrounded by silicon (111) surfaces. With this texture, it is possible to allow repeated reflections a plurality of times in such a manner that sunlight once reflected by the surface of the texture reaches the surface of the texture again, thereby allowing absorption of more sunlight (an optical confinement effect).
During the manufacturing of the solar power generator, the texture is usually formed on the surface of the silicon substrate by immersing the silicon substrate in a high-temperature chemical solution (a wet etchant) obtained by adding, as an additive, organic matter such as IPA to an alkaline aqueous solution such as sodium hydroxide or potassium hydroxide and then by using the property that the etching rate is different depending on the surface orientation of silicon.
In order to improve the quality of the solar power generator, various manufacturing processes performed between a step of slicing and processing a silicon ingot into a substrate and a step of forming the texture structure on the surface of the substrate have been proposed.
When the substrate is sliced, chips and abrasive generated by the scraping of the wire adhere to the substrate. Such chips and the abrasive are cleaned and removed after the slicing. Further, processing strain caused by the slicing, referred to as a “damaged layer”, is generated on the surface layer of the sliced substrate up to a depth of about 5 μm. When the damaged layer remains in the solar power generator, recombination of electrons is accelerated in the damaged layer, causing degradation of the characteristics of the solar power generator. Therefore, it is generally necessary to perform a step of removing the damaged layer.
For removing the damaged layer, a step is required in which the substrate surface is etched by using, for example, an alkali solution or a mixed solution of hydrofluoric acid and nitric acid to scrape the substrate surface. By performing such a step of removing the damaged layer, organic impurities and metal impurities that are contaminants remaining on the substrate surface are removed at the same time as the separation of a natural oxide film.
However, when a silicon active surface is exposed as a result of the removal of the damaged layer, organic impurities and metal impurities hindering formation of the texture easily adhere to the surface of the substrate, and therefore such a substrate cannot be placed in the air for a long time. As a method for preventing adhesion of impurities to the substrate even if the substrate is left in the air for a long time after the removal of the damaged layer, for example, a method is proposed in Patent Literature 1 in which a substrate from which a damaged layer has been removed is immersed in an oxidizing chemical solution to form a chemical oxide film on the substrate surface.
With Patent Literature 1, adhesion of organic impurities and metal impurities that are contaminants can be suppressed by forming a chemical oxide film on the active silicon substrate surface from which the damaged layer has been removed and thereby protecting the active silicon substrate surface. It is also disclosed that, because it is possible to leave the silicon substrate after removal of the damaged layer in the air for a long time, the process can be performed without taking care of the storage for the texture formation at the next step.
In this case, however, a device for removing the damaged layer and a device for forming the texture are required. That is, at least two processing devices are required. Further, a drying process and the like have to be performed during each of the treatments performed by the respective devices, making the process redundant.
In addition, Patent Literature 1 discloses that, although organic impurities and metal impurities also adhere to the oxidation film formed on the surface of the silicon substrate from the air, they are removed together with the oxidation film at the texture formation step, which is the next step, and therefore cause no problem.
However, organic impurities and metal impurities adhering to the oxidation film formed on the surface of the silicon substrate remain in the alkaline aqueous solution together with the oxidation film during wet etching that is performed to form the texture using an alkaline aqueous solution (an etchant). After the etching treatment, the alkali and the additive that have been consumed in a single etching treatment are compensated for, so that the etchant is used a plurality of times. Therefore, the organic impurities and the metal impurities separated from the silicon substrate are accumulated in the etchant and cause degradation of the performance of the etchant. Accordingly, there is a problem in that it becomes necessary to shorten the replacement period of the etchant and this leads to an increase in the chemical solution costs.